Storm HQ

The first snowstorm of meteorological winter is expected across at least part of the region on Saturday.

Low pressure moving into the Tennessee Valley this afternoon will cross the Appalachians tonight and move off the Mid-Atlantic coastline on Saturday. Ahead of it, rain will move into our area tonight, and it could be heavy Saturday morning. Given our long-term rainfall deficit, this is a good thing. However, some localized flooding is possible, especially in areas where the storm drains are covered or clogged by fallen leaves. This is the easy part of the forecast.

As the system moves northeastward, an upper-level disturbance will drop southeastward from Canada. This disturbance and the system to the south will eventually phase, resulting in a fairly strong storm system off the East Coast. There is still some question to the exact track the system takes, but it will likely pass close to or just south of Cape Cod and the Islands. As the system intensifies, it will generate some strong winds, especially along the coast south of Boston. High Wind Watches have been posted along the coast in this area, where sustained winds of 25-35 mph are expected, with some gusts to 50 mph or more possible.

As the storm passes by our longitude, it will turn winds from the northeast to the north and eventually northwest. This will help bring colder air down from the north. The airmass to the north isn’t that cold by December standards, but temperatures will drop below freezing, which will allow for the rain to change over to snow from northwest to southeast as the low passes by. The exact track of the storm will determine exactly where the rain/snow line ends up. A track farther to the north and west results in less snow to the south and east.

When that changeover occurs also will have a big impact on the snowfall amounts. Right now, here’s our thoughts on the changeover timing:

Late morning: The higher terrain from central Massachusetts into southwestern New Hampshire
Midday/Early Afternoon: Southern New Hampshire
Early/Mid Afternoon: Merrimack Valley and New Hampshire Seacoast
Late Afternoon: MetroWest and the North Shore as well as Northern Rhode Island
Early Evening: I-95 corridor from Boston to Providence and parts of Southeastern Massachusetts.

Everything should wind down and end by midnight as the storm moves into the Gulf of Maine and pulls away from the area.

Beyond the timing and track issues, we have one more thing that complicates the snowfall forecast. As the system gets cranked up, bands of very heavy snow will develop. Some of these bands may produce 1-2 inches of snow in an hour. Exactly where those bands set up is nearly impossible to determine in advance, and even trying to figure out a general area for them is tough, because part of it will depend on the track of the system.

Having said all of that, here’s our best estimate right now for snowfall:

Dusting (if that): Cape Cod and the Islands
1-2″: Southeastern Massachusetts and Southern Rhode Island
2-4″: I-95 corridor from Boston to Providence
3-5″: Northern Rhode Island/MetroWest/North Shore/New Hampshire Seacoast
4-7″: Merrimack Valley/Southern New Hampshire
5-9″: Worcester County/Southwestern New Hampshire

Obviously this is a low-confidence forecast, based on all of the factors above, but it’s our best guess at the moment. If there’s any significant changes, we may do another quick update Saturday morning/early afternoon.

The next few days after the storm look calmer, but another system will develop off the East Coast on Tuesday. Right now, it looks like it will stay too far south and east to have any impacts here, but we’ll keep an eye on it.